Miscellaneous active electrical nonlinear devices – circuits – and – Gating – Compensation for variations in external physical values
Reexamination Certificate
1999-12-01
2001-04-17
Tran, Toan (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Gating
Compensation for variations in external physical values
C327S513000, C327S362000
Reexamination Certificate
active
06218886
ABSTRACT:
This invention relates to CMOS analogue integrated circuits and in particular concerns a device for compensating the process and operating parameter variations in such circuits.
A CMOS integrated circuit is manufactured by making use mainly of P channel transistors and N channel transistors. The characteristics and performance of the individual transistors depend on the characteristics of the technological processes that have led to their creation on the integrated circuit as well as on the operating conditions. Thus they may vary from one specimen to another of the same circuit, as well as during the use of a same specimen. Furthermore, the fabrication processes of the two types of MOS transistor differ, thus making their variability ranges partially uncorrelated and quantitatively inhomogeneous.
All this results in a fluctuation of the value of the output quantity of the integrated circuit including the transistors about a mean value. This mean value may represent in practice a more likely value (or the nominal value), corresponding to process and operating conditions that are usually denoted as “typical”.
If, for the sake of clarity, attention is focussed on an integrated circuit whose output quantity is tuneable by acting on the value of an appropriate control quantity, such as for instance a current-controlled oscillator, the variations of the characteristics of the transistors result in a variation of the output quantity (controlled quantity) for a given value of the control quantity. The amplitude of the fluctuations may typically reach ±50% as far as the influence of the process is concerned, and ±30-40% as far as the influence of the operating conditions is concerned. These fluctuations are additive, so that in the worst case a deviation of ±80-90% from the nominal value may take place, and this obviously causes serious problems.
It is therefore clear the interest of manufacturers and users of integrated circuits of this type to have circuits providing (at least with a good approximation) a desired value of the output quantity, regardless of the process results and operating conditions.
It is well known that the compensation of the fluctuations of the output quantity of a device can be obtained through the use of feedback loops that act on a control quantity. Said solution cannot be applied to the compensation of the fluctuations of the output quantity of an integrated device, in particular if this is a tuneable device.
As a matter of fact, on one hand it is desirable to manufacture also the feedback loop as an integrated circuit, and therefore the compensation means would suffer from the same problems of performance fluctuations as the device to be compensated. On the other hand, the feedback loop should be able to distinguish the variations of the output quantity actually due to the process or to the operating conditions from the variations due to a change in tuning, and its intervention should be avoided in the latter case.
According to the invention, a compensation device is provided which also can be implemented as an integrated circuit, preferably together with the device to be subjected to compensation. In particular, the invention applies to tuneable integrated devices wherein the value of an output quantity is determined by the value of a control quantity, and compensates the fluctuations of the output quantity by acting in open loop on the control quantity to bring it to such a value that, given the specific type of integrated circuit and the particular operating conditions, the input-output characteristic remains the one corresponding to process and operating conditions typical for the particular tuning value desired.
The circuit comprises:
means for generating a first compensation signal that depends on quality indexes of the fabrication process of the P transistors and of the N transistors of the device to be compensated, which indexes are determined in a calibration phase of the device, and that is representative, according to an inverse transfer function of the transfer function of the device, of a deviation of the controlled quantity from the desired value, caused by the deviation of said quality indexes from a typical value;
means for generating a second compensation signal representative, according to said inverse transfer function, of a deviation of the controlled quantity from the desired value, caused by the deviation of the operating temperature from a typical value, and
means for simultaneously applying said compensation signals to the control quantity, so as to generate in said control quantity such variations as to bring again the value of the output quantity to the desired value.
Preferably, said means for generating and applying the compensation signals form together with the device to be compensated an open loop structure and are they too manufactured as a CMOS integrated circuit.
So far neither circuits nor methods are known that act in such a way on integrated circuits. The work “Low-Jitter Process-Independent DLL and PLL Based on Self-Biased Techniques” by J. G. Maneatis, IEEE Journal of Solid State Circuits, Vol.31, No 11, November 1996, page 1723 and ff., suggests that the independence of the performance of loop integrated circuits from the process and environmental conditions can be achieved by eliminating the external bias, and making resort to a self-bias through which the device chooses the bias levels that guarantee its best performance. In practice, an action on current mirrors is envisaged so as to vary the current flowing through the circuit.
REFERENCES:
patent: 5303191 (1994-04-01), Eagan et al.
patent: 5477176 (1995-12-01), Chang et al.
patent: 5994945 (1999-11-01), Wu et al.
Balistreri Emanuele
Burzio Marco
CSELT - Centro Studi e Laboratori Telecomunicazioni S.P.A.
Dubno Herbert
Tran Toan
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